Radiation Shielding
August 27, 2015
Exposure to
science fiction films is enough to give people a little knowledge of
physics without the nuisance of getting a physics
education. In this way, nearly everyone knows that
lead will act as a
shield from
radiation, with thick slabs giving the best protection. Our "
atomic age" was aided by the ubiquity of lead, a
metal common in
antiquity.
Lead was produced in antiquity by merely
heating the
mineral,
galena (
lead sulfide (PbS), in
air to produce
lead oxide (PbO). The lead oxide is then
reduced in the presence of
carbon (C) to form free lead; viz.,
2PbS + 3O2 -> 2PbO + 2SO2
PbO + C -> Pb + CO
Lead is a reasonably
abundant element in the Earth's crust. As shown in the table, it's not as abundant as
iron and a few of the other metals, but quite a bit more abundant than
tin,
silver and
gold. Galena is also a source of silver.
It's not that lead has a magical
property among the
elements that enables its shielding effect. All other
materials will shield from radiation, but most to a lesser extent. Lead is most often used for shielding, since it's an inexpensive material with significant shielding power. While other
laboratories strive to remove all lead for
environmental and
health reasons, as I summarized in a
previous article (Leaded Brass, November 29, 2010), the
X-ray diffraction laboratory I used had lead everywhere, in sheets, and as an additive (up to about 30%) to
Plexiglas for
transparent shielding enclosures.
Ample shielding from
X-rays in a typical diffraction laboratory for
crystallography requires just small amounts of lead. That's because the
energy of X-rays is in the range of just a few ten thousand
electronvolts (~10
4 eV). Radiation, however, extends to very high energies.
Gamma rays have energies an
order of magnitude above that of X-rays and far beyond.
Cosmic rays of energy 10
20 eV have been observed.
The
transmittance of radiation through a
material, the measure of how well it acts as a shield, follows the law,
in which
I/I0 is the
ratio of
intensity to initial intensity,
μ/ρ is the
mass attenuation coefficient of the specific material at the specific X-ray energy, and ℓ is the distance through the material. There are tables of
μ/ρ for many materials at various X-ray energies.[1] The mass attenuation coefficient of lead over a wide range of radiation energy is shown in the following graph. As can be seen from the graph, the shielding effectiveness is less at higher energies, so a greater thickness of lead is required.
Humans are not the only objects that need radiation protection.
Electronic components in
spacecraft need radiation shielding, also, along with protection from
impacts from small pieces of
space debris. A
research team at
North Carolina State University led by
Afsaneh Rabiei, a
professor of
mechanical and aerospace engineering, has been investigating
composite metal foams effective at both radiation shielding and absorption of the energy of high impact
collisions.[2-3] I wrote about metal foams in a
recent article (Low-Density Syntactic Foam Alloy, June 18, 2015).
The NCSU team prepared a variety of metal foams and compared their radiation shielding effectiveness at blocking X-rays, gamma rays and
neutron radiation. For an accurate comparison, they normalized their results to the
areal density.[3] The
matrix materials were
316 L stainless steel,
high-speed T15 steel and some
aluminum alloys mixed with 2-, 4- and 5.2- mm hollow steel
spheres. The T15 steel alloy contains high
concentrations of
tungsten and
vanadium and was designated "high-Z foam."[2] Tungsten, because of its high
atomic number of 74, has good shielding ability (lead has the atomic number 82).
Radiation test were performed using
isotopes of
cesium and
cobalt as high energy gamma ray emitters, and isotopes of
barium and
americium as lower-energy gamma ray emitters.[3] The high-Z foam was comparable to bulk materials in its ability to block high-energy gamma rays, but it was much better than even bulk
steel as a shield for low-energy gamma rays.[3] It also was better at blocking neutron and X-ray radiation, but it was not as good as lead as an X-ray shield.[3] It is, however, lighter in
weight and more environmentally friendly than lead.[2]
The effectiveness of the foam's radiation shielding was not affected by the sphere
geometry, as long as the
ratio of wall thickness to the
diameter of the spheres was constant. Small spheres, however, seemed to lead to foams of slightly better shielding.[2]
Quasi-static compression testing showed that the foams have good energy absorption capability.[2] Says Rabiei,
"... We are working to modify the composition of the metal foam to be even more effective than lead at blocking X-rays - and our early results are promising... and our foams have the advantage of being non-toxic, which means that they are easier to manufacture and recycle. In addition, the extraordinary mechanical and thermal properties of composite metal foams, and their energy absorption capabilities, make the material a good candidate for various nuclear structural applications."[3]
This work was supported by the
US Department of Energy's Office of Nuclear Energy.[2]
References:
- X-Ray Mass Attenuation Coefficients on NIST web site.
- Shuo Chen, Mohamed Bourham, and Afsaneh Rabiei, "Attenuation efficiency of X-ray and comparison to gamma ray and neutrons in composite metal foams," Radiation Physics and Chemistry, Early Online Publication, July 8, 2015.
- Matt Shipman, "Study Finds Metal Foams Capable of Shielding X-rays, Gamma Rays, Neutron Radiation," North Carolina State University Press Release, July 17, 2015.
